Real-time rendering of large-scale 3D scenes is important in computer graphics and virtual reality technology. One major challenge is that the large-scale scene may have high geometric complexities. Natural scenes in real-life contain various objects with rich geometric details and complex shapes. A typical complex scene is the plant model. The plant model has a very complex structure, which bring great difficulties in real-time and realistic rendering of the scenes. For example, assuming that a forest scene comprises 15,000 tree models, each of which comprising about 50,000 polygons in average, the scene will comprises a total number of 750 million polygons. It is very difficult to render a scene with such high geometric complexities in real-time while maintaining a certain degree of reality.
“Billboards” method is a common method in rending complex models. It is an Image-based rendering (TBR) method. It represents a model with a semi-transparent texture image oriented toward a view point or a plurality of intersecting texture images, so as to simplify the model. For the plant model, this method is very effective for the model distant from the view point because a projection of a leaf of the distant plant on a screen usually has a size less than a pixel. Due to the simple geometric structure of the plant represented by the Billboards method, this method has a very fast rendering speed. However, this method has an inferior effect of binocular parallax and may generate errors for bird's eye-view scenes. Decoret et al. proposed a “Billboard Clouds” method in 2003 (Decoret, X., Durand, F., Sillion, F. X., and Dorsey, J. 2003. Billboard Clouds for Extreme Model Simplification. ACM Trans. Graph. 22, 3, 689-696). This method simplifies any mesh model using billboard clouds. According to this method, geometric information of the model is projected into another space. An optimal set of billboards are found in the other space using Greedy algorithm to represent an input object with minimum errors. This set of billboards may have arbitrary orientations and positions, which is advantageous over conventional billboards. However, rendering plants using this method directly generally does not achieve idealistic effects. This method has been improved for simplification of plant models with more complex geometric structures. Fuhrmann et al. proposed an improvement of the method proposed by Decoret et al. (Fuhrmann A., Umlauf, E., Mantler, S.: Extreme Model Simplification for Forest Render, in Eurographics Workshop on Natural Phenomena 2005, 57-66). This method generally determines a series of sample view points in preprocessing. In the direction of each view point, the plant is rendered and rendering results are saved as texture images. In real-time rendering, for a current view point, nearest-neighbor sample view points are found according to information of the current sample view point. Then texture images corresponding to these sample view points are interpolated to generate rendering results for the current view point. Behrendt et al. proposed in 2005 a better approximation method by improving the Billboard Clouds method with reference to plant topology using heuristic clustering. (Behrendt, S., C. Colditz, O. Franzke, J. Kopf, and O. Deussen. 2005. Realistic Real-Time Rendering of Landscapes Using Billboard Clouds. Computer Graphics Forum. 24, 3, 507-516). The Billboards method can substantially reduce the geometric complexity of the plant model. However, it is difficult to maintain the appearance of the plant. The geometry is presented based on images, so that it is usually different to obtain satisfying effect of model rendering.
Later on, a method has been proposed by simplify the plant model by combining geometric mesh and Billboards, i.e., a combination of 2D and 3D methods. For example, Bao et al. simplify leaves of the plant using the Billboard Clouds method and maintain the geometric shape of the items to obtain better parallax than simply using the Billboards method. (Guanbo Bao, Xiaopeng Zhang, Wujun Che, Jaeger, M. 2009. Billboards for Tree Simplification and Real-Time Forest Rendering. PMA '09 Proceedings of the 2009 Plant Growth Modeling, Simulation, Visualization, and Applications, 433-440). However, the combination of 2D and 3D methods does not greatly improve the rendering effect. It is still difficult to calculate light illumination of the billboards obtained by simplifying the leaves. Decaudin et al. proposed an improved method for rendering the complex scenes in 2004 (Decaudin, P. and Neyret, F. Rendering Forest Scenes in Real-Time. 2004. In Proc. of the 2004 Eurographics Workshop on Rendering, 93-102) and in 2009 (Decaudin, P. and Neyret, F. 2009. Volumetric Billboards. Computer Graphics Forum, 28, 8, 2079-2089). In rendering the scene, the scene model is first rendered as 3D texcells and then the texcells are laid on geography non-periodically to rendering the scene. This method has a fast rendering speed and is suitable for plan-view observation such as in aircraft simulation. However, the observer cannot roam in the scene. Also, this method using the texcells consumes a large amount of memory resource of the computer. With development of computer hardware, people are putting higher requirement on the reality of the large-scale complex scene. However, neither the Billboards method nor the texcell method provides idealistic reality in rendering the scene. An object rendered using the Billboards method has a “thin-slice” appearance when it is viewed from a short distance. It lacks a 3D effect and the shadow effect is inferior. The light illumination in the texcell method is pre-calculated and thus dynamic illumination and shadow effect cannot be achieved when the scene is rendered. The rendered scene appears non-realistic due to excessive repetitiveness.
Deng et al. proposed a method of leaves simplification in GPU using pure 3D mesh model. (Qingqiong Deng, Xiaopeng Zhang, Gang Yang, Marc Jaeger. 2010, Multiresolution Foliage for Forest Rendering, Journal Computer Animation and Virtual World, 21, 1, 1-23). The plant model obtained by this simplification method is suitable for complex calculation of illumination shadow and its rendering effect is better than the above-mentioned methods. In view of this, the model sequence generated by this simplification method will be employed in the present invention.
Shadow is one of the most basic elements in nature world. Proper utilization of shadows may greatly enhance reality of the scene. However, the shadows of the large-scale scene often have obvious sawtooth effect. Zhang et al. proposed Parallel-Split Shadow Maps (PSSM) in 2006 (Fan Zhang, Hanqiu Sun, Leilei Xu, Lee Kit Lun, Parallel-Split Shadow Maps for Large-Scale Virtual Environments, Proceedings of the 2006 ACM international conference on Virtual reality continuum and its applications, June 14-Apr. 17, 2006, Hong Kong, China). According to this method, a frustum of a viewpoint is divided into a plurality of parallel partitions according to a predetermined rule. A shadow map is generated using depth textures of equal size for each part. In this way, the sample rate for the screen is the same as that for the shadow map, resulting in a good anti-aliasing effect. This method is widely used in generating shadows of the large-scale scene. However, this method cannot realize real-time realistic rendering of the large-scale complex scene because it is not combined with model simplification.